1. Field of the Invention
The present invention relates to a temperature sensing type flow sensor for measuring the amount of air taken in an engine.
2. Description of the Related Art
Conventional temperature sensing type flow sensors of an engine control digital output type have been disclosed in Japanese Patent Application Laid-Open Nos. 56-51619, 62-79316, and 62-55518. FIG. 1 schematically illustrates a circuit used in a digital signal output type temperature sensing flow sensor disclosed in Japanese Patent Application Laid-Open No. 62-55518 wherein the amount of air taken in an engine is measured.
Referring to FIG. 1, a temperature sensing element 2 for heating and a temperature sensing element 3 for detecting the temperature of air are provided in an intake pipe 1. The temperature sensing elements 2 and 3 each comprise a thin wire or a thin film resistor element made of, for example, platinum, tungsten or nickel whose resistance can be changed, and they are arranged to be positioned in contact with the air flow through the intake pipe 1. An Wheatstone bridge circuit is constituted by these temperature sensing elements 2 and 3 and fixed resistors 4 to 6. A heating electricity is supplied, via a transistor 25, to this bridge circuit. The output from the bridge circuit is connected to a comparator 20. An output signal from the comparator 20 is supplied when the temperature of the temperature sensing element 2 has been raised by the heating electricity to a level which is different by a predetermined degree from that of the temperature sensing element 3 for detecting the temperature of air. The output signal from this comparator 20 resets a flip-flop 21, and the output from this flip-flop 21 makes the switch element 23 close via a buffer 22. The voltage of the electricity to be supplied from the transistor 25 to the temperature sensing element 2 is controlled at a constant level with reference to the reference voltage power source 26.
In this structure, the flip-flop 21 is set in response to a start pulse signal, and the flow of heating current is turned on so that the temperature sensing element 2 is heated. When the temperature of the temperature sensing element 2 has been raised to a level which is different by a predetermined degree from that of air, an output signal is generated by the comparator 20 so that the flip-flop 21 is reset and the switch element 23 is closed, causing the transistor 25 to be grounded. As a result, the heating current is stopped. The speed at which the temperature of the temperature sensing element 2 rises is arranged to be in inverse proportion to the air flow though the intake pipe 1. Therefore, the time taken from the start of the flow of heating current to the generation of the output signal supplied from the comparator 20 is caused to correspond to the flow of the intake air passing though the intake pipe 1. Therefore, by measuring the time period of the output pulse from the flip-flop 21, the amount of intake air can be measured.
The conventional digital signal output type temperature sensing type flow sensors are structured as described above in which the time taken for the difference between the temperature of the sensing elements 2 and that of 3 to become a predetermined level is measured. Therefore, only poor responsiveness can be realized with respect to that obtained with the fixed temperature method, and since the temperature of the temperature sensing element 2 can be rapidly changed, the resistance-temperature characteristics of the temperature sensing element 2 are changed in accordance with the elapse of time.